Electrolyte adaptive to dry-method electrode plate, preparation method of electrolyte and lithium ion battery

Abstract

The invention provides an electrolyte adaptive to a dry-method electrode plate, a preparation method of the electrolyte and a lithium ion battery. Wherein the electrolyte adaptive to the dry-method electrode plate comprises an organic solvent and a lithium salt, and the electrolyte adaptive to the dry-method electrode plate further comprises 1%-5% of a first additive, 2%-6% of a second additive and 1%-3% of a third additive by taking the total weight of the electrolyte adaptive to the dry-method electrode plate as 100%; the lithium salt is LiPF6; the LUMO energy levels of the first additive, the second additive, the third additive and LiPF6 are ranked as follows: the first additive < the second additive < LiPF6 < the third additive. By accurately controlling the dosage of the three special additives and the LUMO energy level, the side reaction caused by a dry-method electrode in a battery system is remarkably inhibited.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to an electrolyte adapted to dry-process electrode sheets, a preparation method thereof, and a lithium ion battery. Background Art

[0002] At present, there are two main methods for preparing electrodes for lithium-ion batteries: dry method and wet method. The wet method is to mix the active material with solvents, conductive agents, binders, etc. at room temperature, then extrude and roll-form, and then dry at high temperature to prepare electrode sheets. The wet method is the current mainstream electrode preparation method. With the improvement of traditional wet battery technology, the performance improvement in both materials and pole sheet levels has basically reached the upper limit bottleneck, and it is difficult to make a major breakthrough. Faced with the increasing requirements for voltage, capacity, etc., researchers have gradually shifted their attention to dry electrodes. Dry electrode technology is an electrode sheet preparation process in which active materials, conductive agents and binders are mixed at high speed to obtain fiberized electrode powder, which is then formed by continuous hot rolling, and then thermally compounded with the current collector to obtain electrode pole sheets.

[0003] The advantages of the dry process for manufacturing battery pole pieces are high porosity, good conductivity, good electrochemical stability and safety, high energy density, and long service life. At the same time, the dry process for preparing pole pieces has low production costs, higher production efficiency, and no organic solvents are used in the process of preparing pole pieces, which is extremely environmentally friendly and can significantly reduce the cost of preparing lithium-ion batteries. However, since binders are inevitably used in the process of dry manufacturing battery pole pieces, commonly used binders, especially polytetrafluoroethylene (PTFE), will produce side reactions on the pole pieces, especially on the negative pole pieces, thereby affecting the capacity and cycle performance of lithium-ion batteries.

[0004] Based on this, how to develop electrolyte for electrode sheets obtained by dry manufacturing so that it can better adapt to electrode sheets prepared by dry manufacturing, reduce the side reactions caused by binders in dry electrodes, and thus make the corresponding lithium-ion batteries show higher first charge and discharge efficiency and cycle capacity retention rate, is one of the important technical problems that need to be solved in this field. Summary of the invention

[0005] The main purpose of the present invention is to provide an electrolyte suitable for dry-process electrode sheets, a preparation method thereof and a lithium-ion battery, so as to solve the problem of poor initial efficiency and cycle stability of lithium-ion batteries caused by side reactions brought about by binders in dry-process electrodes in lithium-ion batteries where dry-process electrode sheets are used in the prior art.

[0006] In order to achieve the above-mentioned object, the first aspect of the present invention provides an electrolyte adapted to a dry electrode sheet, comprising an organic solvent and a lithium salt, wherein the total weight of the electrolyte adapted to the dry electrode sheet is 100%, the electrolyte adapted to the dry electrode sheet further comprises 1% to 5% of a first additive, 2% to 6% of a second additive, and 1% to 3% of a third additive; the lithium salt is LiPF 6 ; The first additive, the second additive, the third additive and LiPF 6 The LUMO energy level order is: first additive < second additive < LiPF 6 <Third additive.

[0007] Furthermore, the first additive is selected from one or more of electrolyte additives and / or sulfonimide additives; the second additive is selected from one or more of organic ester additives; and the third additive is selected from one or more of electrolyte additives.

[0008] Furthermore, the weight ratio of the first additive, the second additive and the third additive is (0.5-2.5):2:1.

[0009] Further, the first additive is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium diethyl oxalate, N-phenylbis(trifluoromethanesulfonyl)imide, N-methylbis(trifluoromethanesulfonyl)imide and N-ethylbis(trifluoromethanesulfonyl)imide.

[0010] Further, the second additive is selected from one or more of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, di(alkyne) oxalate and methylene methanedisulfonate.

[0011] Furthermore, the third additive is selected from one or more of lithium difluorophosphate, lithium chloride, lithium phosphate and lithium carbonate.

[0012] Further, the organic solvent is selected from one or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate and butylene carbonate.

[0013] The second aspect of the present invention provides a method for preparing the above-mentioned electrolyte suitable for dry-process electrode sheets, comprising: step S1, preparing an organic solvent and a lithium salt into a lithium salt solution; step S2, adding a first additive to the lithium salt solution, and obtaining a first mixed solution after a first stirring; step S3, adding a second additive and a third additive to the first mixed solution, and obtaining an electrolyte suitable for dry-process electrode sheets after a second stirring.

[0014] Further, the first stirring rotation speed is 100 rpm to 200 rpm, and the time is 20 min to 30 min; and / or the second stirring rotation speed is 400 rpm to 500 rpm, and the time is 30 min to 40 min.

[0015] The third aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the electrolyte is the electrolyte adapted for the above-mentioned dry-process electrode sheet, and both the positive electrode sheet and the negative electrode sheet are dry-process electrode sheets.

[0016] By applying the technical solution of the present invention, the side reactions caused by the dry electrode in the battery system are significantly suppressed by precisely controlling the dosage of the three special additives and the LUMO energy level. The synergistic effect of the first additive and the second additive effectively promotes the formation of the SEI film; while the third additive helps to maintain the chemical stability of the electrolyte, reduce the occurrence of side reactions, and ultimately significantly improve the first efficiency and cycle stability of the corresponding lithium-ion battery. DETAILED DESCRIPTION

[0017] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0018] As described in the background technology, in the lithium-ion battery where the battery electrode sheet obtained by dry manufacturing in the prior art (hereinafter referred to as dry electrode sheet) is located, there are problems of poor initial efficiency and cycle stability of the lithium-ion battery due to the side reactions caused by the binder in the dry electrode. In order to solve the above technical problems, the first aspect of the present invention provides an electrolyte adapted to dry electrode sheets, including an organic solvent and a lithium salt. Taking the total weight of the electrolyte adapted to the dry electrode sheet as 100%, the electrolyte adapted to the dry electrode sheet also includes 1% to 5% of a first additive, 2% to 6% of a second additive, and 1% to 3% of a third additive; the lithium salt is LiPF 6 ; The first additive, the second additive, the third additive and LiPF 6 The LUMO energy level order is: first additive < second additive < LiPF 6 <Third additive.

[0019] The present invention not only effectively inhibits the side reaction of the binder in the dry electrode sheet and reduces the loss of active lithium during the use of the battery by precisely controlling the LUMO energy level order and content of the three additives, but also optimizes the formation process of the SEI film, ultimately significantly improving the initial charge and discharge efficiency and cycle performance of the lithium-ion battery. Specifically:

[0020] The first additive and the second additive, both of which have LUMO energy levels lower than LiPF 6 The selection and ratio of the electrolyte ensure that they can take precedence over the lithium salt LiPF during the first charge and discharge process of the battery. 6 Decompose to form SEI film. Especially in the battery system with dry electrode sheets, the first additive has a lower LUMO energy level and preferentially forms a film at a lower potential, which can effectively wrap the binder in the dry electrode sheet to prevent it from reacting with lithium ions and reduce the loss of active lithium. The LUMO energy level of the second additive is higher than that of the first additive but lower than that of LiPF 6 , can further modify and optimize the structure of the SEI film after it is formed, improve the density and stability of the film, reduce the impedance of the SEI film, and play a key role in improving the battery cycle performance. 6 The addition of the third additive means that it will not preferentially decompose to form the SEI film, but will participate in the subsequent modification and stabilization of the SEI film, which can effectively improve the elasticity of the SEI film and thus help improve the battery cycle performance.

[0021] The precise control of the concentrations of the above three additives directly affects the formation and properties of the SEI film. When the concentration of the first additive is set at about 1%~5%, a LiF-rich SEI film can be quickly formed, effectively inhibiting the side reactions of the binder in the dry electrode sheet and reducing the loss of active lithium. When the concentration of the second additive is set at 2%~6%, it can be optimized after the SEI film is formed to generate a dense and stable SEI film, further improving the stability and ionic conductivity of the film. When the concentration of the third additive is set between 1%~3%, it helps to improve the elasticity of the SEI film, ensure the appropriate thickness and structure of the film, and avoid excessively thick films that increase the internal resistance of the battery and affect battery performance. By adjusting the concentration of the additive, the best balance can be found between the quality of the SEI film and the overall performance of the battery, and ultimately achieve a significant improvement in the first effect and cycle stability of the corresponding lithium-ion battery.

[0022] Furthermore, with regard to the content of the above three additives, it is preferred that the total weight of the electrolyte adapted to the dry electrode sheet is 100%, the content of the first additive is 1.8%~2%, the content of the second additive is 3%~4%, and the content of the third additive is 1.5%~2%, so as to more effectively realize the effective control of the SEI film formation, optimization, and maturation process in the corresponding battery application process, while improving the initial charge and discharge efficiency of the battery where the dry electrode is located, and better balance the cycle stability, cost, production efficiency and environmental adaptability.

[0023] In several preferred embodiments, the first additive is selected from one or more of electrolyte additives and / or sulfonimide additives; the second additive is selected from one or more of organic ester additives; and the third additive is selected from one or more of electrolyte additives. The present invention uses the difference in LUMO energy levels to allow the first, second and third additives to undergo film-forming reactions, SEI film optimization and SEI film maturation processes in the electrolyte in sequence. Further optimizing the specific types of the three on this basis can promote the formation of a more orderly chemical reaction sequence, more effectively solve the side reaction problem of dry electrode sheets in batteries, thereby more significantly improving the stability, ionic conductivity and chemical compatibility of the SEI film formed during the application process, and ultimately more significantly enhancing the initial charge and discharge efficiency and cycle stability of the corresponding battery.

[0024] In order to promote the first and second additives to form SEI film preferentially during the formation process of the lithium-ion battery in which the electrolyte is located, and the third additive plays a role in the stability and maturity of the SEI film, thereby achieving better battery performance, the weight ratio of the first additive, the second additive and the third additive is preferably (0.5~2.5):2:1. In order to more effectively balance the formation speed and maturity of the SEI film, promote the SEI film to form quickly to protect the electrode, and not to be too thick too early to increase the internal resistance of the battery, thereby achieving better results in the first charge and discharge efficiency and cycle stability, the weight ratio of the first additive, the second additive and the third additive is preferably (1~1.2):2:1.

[0025] In several preferred embodiments, the first additive is selected from one or more of lithium bis(oxalatoborate) (LiBOB), lithium trifluoromethanesulfonate (LiTFSI), lithium diethyl oxalate, N-phenylbis(trifluoromethanesulfonyl)imide (PTFSI), N-methylbis(trifluoromethanesulfonyl)imide and N-ethylbis(trifluoromethanesulfonyl)imide. Preferably, the first additive is lithium bis(oxalatoborate) and / or N-phenylbis(trifluoromethanesulfonyl)imide because, compared with other electrolyte and sulfonyl imide additives, these two can quickly form a more stable SEI film at a lower potential, thereby more effectively inhibiting the side reaction of the binder in the dry electrode, reducing the loss of active lithium, and thus more significantly improving the initial charge and discharge efficiency and cycle life of the obtained lithium ion battery.

[0026] In a particularly preferred embodiment, the first additive is a mixture of lithium bis(oxalatoborate) and N-phenylbis(trifluoromethanesulfonyl)imide, and in the mixture, the weight ratio of lithium bis(oxalatoborate) and N-phenylbis(trifluoromethanesulfonyl)imide is 1:(0.8~1). Mixing the two in a specific weight ratio as the first additive can enable lithium bis(oxalatoborate) to form a SEI film rich in BO compounds on the electrode surface more quickly, providing initial protection and mechanical strength; while N-phenylbis(trifluoromethanesulfonyl)imide can supplement the formation of a SEI film rich in LiF and lithium sulfonate, improving the ionic conductivity and thermal stability of the film. Under this dosage relationship, the synergistic effect of the two additives in the SEI film formation process can be more significant, and ultimately more significantly improve the initial charge and discharge efficiency and cycle stability of the corresponding lithium-ion battery.

[0027] Further, the second additive is preferably selected from one or more of vinylene carbonate (VC), vinyl sulfate (DTD), fluoroethylene carbonate (FEC), di(alkyne) oxalate (BPO) and methylene disulfonate (MMDS), so as to preferentially form a film at a relatively low potential. The second additive is preferably a mixture of vinylene carbonate and vinyl sulfate, and the weight ratio of vinylene carbonate to vinyl sulfate in the mixture is (2~3):1, because the mixing of VC and DTD can produce a synergistic effect, wherein VC helps to form a strong and stable SEI film, and the addition of DTD can reduce the SEI film impedance, improve the SEI film ion conductivity, and thus improve the overall efficiency and life of the battery. For comprehensive consideration of battery performance, it is preferred that the two are mixed in the above ratio, and at this mixing ratio, the properties of the SEI film are more optimized, which not only improves the first charge and discharge efficiency, but also enhances the cycle stability.

[0028] In several preferred embodiments, the third additive is selected from lithium difluorophosphate (LiPO 2 F 2 )、lithium chloride (LiCl), lithium phosphate (Li 3 PO 4 ) and lithium carbonate (Li 2 CO 3 ) or more. More preferably, it is LiPO 2 F 2 , because LiPO 2 F 2 There is still a synergistic effect with the VC of the first additive, forming a VC-rich, Li-rich 2 C 2 O 4 、LiF、Li 3 PO 4The inorganic-organic hybrid interface membrane can effectively improve the elasticity of the membrane, thereby significantly improving the stability of the membrane, reducing the incidence of side reactions, and ultimately improving the cycle performance and energy density of the lithium-ion battery in which the electrolyte is located.

[0029] For the organic solvent in the electrolyte system, it is preferably selected from one or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate and butylene carbonate. The preferred organic solvent is selected from at least two of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate, because the use of EC, EMC and DMC in pairs can better balance the dielectric constant and viscosity of the electrolyte, optimize the transmission efficiency of lithium ions, and form an electrolyte with better ionic conductivity and chemical stability. More preferably, the organic solvent is a mixed solvent formed by ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate, and in the mixed solvent, the volume ratio of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate is (0.7~0.8): (0.7~0.8): 1. The adjustment of the above volume ratio can better play the role of the higher dielectric constant of EC in promoting the dissolution of lithium ions, as well as the lower viscosity and good lithium ion conductivity of EMC and DMC, so as to improve fluidity. In particular, it promotes the improvement of lithium ion transmission efficiency and the ability of the three additives to form SEI film, ultimately significantly improving the initial efficiency and cycle life of the battery in which they are located.

[0030] After a lot of experiments, the inventors found that in several particularly preferred embodiments, the electrolyte suitable for the dry electrode sheet preferably includes 1% lithium bis(oxalatoborate), 1% N-phenylbis(trifluoromethanesulfonyl)imide, 3% vinylene carbonate, 1% vinyl sulfate and 2% lithium difluorophosphate, based on the total weight of the electrolyte suitable for the dry electrode sheet as 100%; or, the electrolyte suitable for the dry electrode sheet preferably includes 1. 6% lithium bis(oxalatoborate), 0.4% N-phenylbis(trifluoromethanesulfonyl)imide, 3% vinylene carbonate, 1% vinyl sulfate and 2% lithium difluorophosphate; or, based on the total weight of the electrolyte adapted to the dry electrode sheet as 100%, the electrolyte adapted to the dry electrode sheet includes 0.8% lithium bis(oxalatoborate), 1.2% N-phenylbis(trifluoromethanesulfonyl)imide, 3% vinylene carbonate, 1% vinyl sulfate and 2% lithium difluorophosphate. Based on a large amount of experimental data analysis, when the types and concentrations of the above components are optimized, the problem of binder side reactions in dry electrode batteries can be particularly effectively solved, thereby significantly improving the comprehensive performance of the lithium-ion battery in which the dry electrode is located, including the improvement of the first coulomb efficiency and the enhancement of cycle stability.

[0031] The second aspect of the present invention provides a method for preparing the above-mentioned electrolyte adapted to the dry electrode sheet, comprising: step S1, preparing an organic solvent and a lithium salt into a lithium salt solution; step S2, adding a first additive to the lithium salt solution, and obtaining a first mixed solution after a first stirring; step S3, adding a second additive and a third additive to the first mixed solution, and obtaining an electrolyte adapted to the dry electrode sheet after a second stirring. Step S1 creates the basic framework of the electrolyte, providing a more compatible environment for the subsequent addition of the three additives. Step S2 enables the first additive to be more fully dissolved and evenly dispersed in the lithium salt solution in advance, avoiding local concentrations that are too high or too low and affecting the performance of the resulting electrolyte. The second stirring in step S3 further promotes the full mixing of all additives, making the film formation in the subsequent application process more controllable, thereby more effectively improving the initial coulombic efficiency and cycle life of the lithium-ion battery during application.

[0032] Furthermore, in order to promote the uniform dissolution of the first additive, reduce bubbles and local overheating that may be caused by high-speed stirring, and reduce the mechanical damage of the additive, so that the first additive can maintain its chemical activity while being evenly dispersed, the first stirring speed is preferably 100rpm~200rpm, and the time is 20min~30min. For the second stirring process, the second stirring speed is preferably 400rpm~500rpm, and the time is 30min~40min, so as to further refine the distribution of components in the electrolyte, promote all additives to merge into the lithium salt solution, form a more uniform and stable electrolyte system, and reduce the loss of additives. Furthermore, the formation conditions of the SEI film in the subsequent battery formation process are better, and ultimately more effectively improve the various performances of the lithium-ion battery in which it is located.

[0033] Regarding the preparation process of electrolysis, under low temperature conditions, the solubility of lithium salts decreases, and the undissolved lithium salt particles lead to uneven local concentrations in the electrolyte, which reduces the ion transfer rate of the electrolyte, resulting in poor first-effect and cycle performance; under high temperature conditions, lithium salts are easily decomposed to form hydrofluoric acid (HF) to corrode electrode materials, causing obvious adverse effects on the battery. Therefore, in several typical implementations, the first stirring and the second stirring are independently carried out at 25°C~30°C to provide more suitable thermodynamic conditions, while accelerating the dissolution of each additive and improving the uniformity of the electrolyte, reducing the side reactions that may be caused by overheating, thereby more effectively maintaining the activity of the additives and the stability of the electrolyte.

[0034] In several more typical embodiments, step S1, step S2 and step S3 are all carried out under the conditions of water content <0.1ppm and oxygen content <0.1ppm. Because water and oxygen are the main pollutants in the preparation of the electrolyte, their presence will trigger a chain reaction, causing the hydrolysis of lithium salts, additives, etc. to generate HF, corroding the materials, and the additives are oxidized to form an SEI film structure that is too loose, thereby degrading the performance of the electrolyte and the battery. By optimizing the environmental conditions in the process of preparing the electrolyte as above, the purity of the electrolyte components, especially the additives therein, can be effectively improved, thereby improving the cycle stability and safety of the lithium-ion battery.

[0035] The third aspect of the present invention provides a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the electrolyte is the above-mentioned electrolyte adapted to the dry electrode sheet, and both the positive electrode sheet and the negative electrode sheet are dry electrode sheets. Due to the use of the above-mentioned electrolyte with excellent performance and adapted to the dry electrode sheet, the low LUMO energy level additive therein can preferentially form a film during the battery formation process, wrap the PTFE binder, reduce its direct contact with lithium ions, thereby inhibiting the side reaction of the binder, and significantly improving the battery's initial charge and discharge efficiency. In the subsequent charging stage, the high LUMO energy level additive modifies and supplements the SEI film that can be formed at a low potential, generating a strong and moderately thick SEI film, and ultimately comprehensively improving the battery's initial coulomb efficiency and cycle life.

[0036] In practical applications, the positive electrode sheet and the negative electrode sheet each independently include an electrode active layer and a current collector, and the electrode active layer is prepared by mixing, dry forming and rolling an electrode active material, a conductive agent and a binder in sequence.

[0037] In particular, when the binder in the preferred positive electrode sheet and the negative electrode sheet is polytetrafluoroethylene, it means that the potential risk of side reactions in the entire battery system increases. By adopting the above-mentioned electrolyte provided by the present invention, it is possible to more effectively control the side reactions caused by PTFE without affecting other battery properties, and significantly improve the comprehensive performance of the battery, including capacity retention, coulomb efficiency and cycle life.

[0038] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0039] Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0040] Example 1

[0041] Preparation of an electrolyte suitable for dry electrode sheets:

[0042] The following steps were carried out in an argon-filled glove box.

[0043] Environmental conditions: moisture content <0.1ppm, oxygen content <0.1ppm.

[0044] Step (1), ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 3:3:4 for about 30 minutes to obtain an organic solvent, and then 1 mol / L lithium hexafluorophosphate (LiPF6) was slowly added to the organic solvent. 6 ) and stirred for 30 min to form a lithium salt solution.

[0045] Step (2), adding N-phenylbis(trifluoromethanesulfonyl)imide (PTFSI), i.e., a first additive, to the lithium salt solution, and stirring the mixture at 25° C. at a rotation speed of 200 rpm for 30 min to obtain a first mixed solution;

[0046] Step (3), adding vinylene carbonate (VC) and vinyl sulfate (DTD) (the second additive) and lithium difluorophosphate (LiPO4) to the first mixed solution. 2 F 2 ) i.e. the third additive, is stirred for a second time at 25°C with a rotation speed of 400 rpm for 30 min to obtain an electrolyte suitable for dry-process electrode sheets.

[0047] In the obtained electrolyte, based on the total weight of the electrolyte being 100%, the content of each additive is shown in Table 1.

[0048] Preparation of a lithium-ion battery:

[0049] Dry preparation of positive electrode sheet:

[0050] The positive electrode active material lithium iron phosphate (LiFePO4), conductive agent SP and binder polytetrafluoroethylene (PTFE) are placed in a stirring tank according to a weight ratio of 96:2:2. The high-speed shear force generated by high-intensity stirring is used to evenly disperse the materials to obtain a fibrous powder. Then, a pole piece film with a thickness of 160μm is obtained by extrusion molding and rolling. After being compounded with an aluminum foil current collector, a dry electrode positive pole piece for a lithium-ion battery is obtained.

[0051] Dry preparation of negative electrode sheet:

[0052] The negative electrode active material graphite, the conductive agent carbon nanotube (CNT) and the binder polytetrafluoroethylene (PTFE) are placed in a stirring tank according to a weight ratio of 97.7:0.8:1.5. The high-speed shear force generated by high-intensity stirring is used to evenly disperse the materials to obtain a fibrous powder. Then, a pole piece film with a thickness of 100 μm is obtained by extrusion molding and rolling. After being compounded with a copper foil current collector, a lithium-ion battery dry electrode negative pole piece is obtained.

[0053] Battery cell assembly:

[0054] The positive electrode sheet, diaphragm PE, and negative electrode sheet prepared above are stacked in order, with the diaphragm placed between the positive and negative electrode sheets, and the sheets are stacked to obtain a bare battery cell; the bare battery cell is placed in an aluminum-plastic film outer packaging to obtain a soft-pack battery sample to be filled with liquid; the electrolyte adapted to the dry-process electrode sheet obtained above is injected therein to obtain a lithium-ion battery sample with a capacity of 5Ah.

[0055] The difference between Examples 2 to 12 and Comparative Examples 1 to 5 and Example 1 is only the type and amount of each additive, see Table 2 for details.

[0056] Example 13

[0057] Preparation of an electrolyte suitable for dry electrode sheets:

[0058] The only difference between this embodiment and embodiment 1 is that the volume ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in the organic solvent prepared in step (1) is changed to 1:1:2.

[0059] Table 1

[0060]

[0061] In Table 1, the three additives used are related to the lithium salt LiPF 6 The LUMO energy levels are ranked as follows:

[0062] The first additive (PTFSI, LiBOB) < the second additive (VC, DTD) < LiPF 6 <Third additive (LiPO 2 F 2 ).

[0063] Electrical performance test method of battery samples

[0064] First effect: At 25°C, charge the battery to 2.8V with a constant current of 0.05C, let it stand for 5 minutes, then charge it to 3.3V with a constant current of 0.2C, let it stand for 12 hours, this is the formation stage; then charge the battery that has been standing for 12 hours with a constant current and constant voltage of 0.2C to 3.65V, let it stand for 5 minutes, and then discharge it to 2.5V with a constant current of 0.2C to complete the battery capacity division. The sum of the capacities of the formation and the first step of the capacity division is the first charge capacity of the battery; the capacity of the capacity division discharge step is the first discharge capacity of the whole battery. Therefore:

[0065] First efficiency = discharge capacity in the second step of capacity division / (charge capacity in formation + charge capacity in the first step of capacity division) × 100%.

[0066] Cycle stability: At 25°C, each battery sample after capacity division was charged to 3.65V at 0.5C constant current, with a cut-off rate of 0.05C; then discharged to 2.5V at 0.5C constant current. After such charge / discharge cycles, the capacity retention rate after the target number of cycles was calculated to evaluate its cycle performance. The target number of cycles was 200.

[0067] The above test results are shown in Table 2.

[0068] Table 2

[0069]

[0070] From the above description, it can be seen that the above embodiments of the present invention achieve the preparation of an electrolyte suitable for dry-process electrode sheets with excellent performance. The LUMO energy level sorting and content control of the three additives not only effectively inhibit the side reaction of the binder in the dry-process electrode sheet and reduce the loss of active lithium during the use of the battery, but also optimize the formation process of the SEI film, and ultimately significantly improve the initial charge and discharge efficiency and cycle performance of the lithium-ion battery.

[0071] Specifically, as shown in the performance results in the table:

[0072] According to the performance data comparison between each embodiment and comparative example 1, the introduction of low LUMO energy level additive (PTFSI / LiBOB) can effectively improve the first coulombic efficiency and cycle stability. This may be because PTFSI / LiBOB preferentially generates LiF-rich lithium oxalate (LiC 2 O 4 ), boron oxide compounds (BO) and other products wrap PTFE, reducing the binder and Li + Direct contact with the battery can inhibit the loss of active lithium caused by the reduction and decomposition of the binder and improve the overall performance of the battery.

[0073] Among them, it can be seen from Comparative Examples 1 to 3 that the first additive improves the battery performance, and the second additive can provide organic-inorganic products to further enrich the SEI film interface. Therefore, the battery performance of Comparative Example 3 is better than that of Comparative Example 2.

[0074] Comparative Example 4 is the extreme value of the second and third additives. Excessive content of the second and third additives will cause excessive reduction and decomposition of the battery in the formation stage, and the formed SEI film will be too thick, seriously affecting the battery's initial charge and discharge efficiency; Comparative Example 5 is the extreme value of the first additive. Its content is too high, causing the battery's internal resistance to increase, affecting the ion transfer rate, and increasing the battery polarization, significantly reducing the battery's initial efficiency and cycle performance.

[0075] The performance differences of each embodiment reflect the influence of different additive types and dosages. Comparing Embodiments 1 to 6 with Embodiments 7 to 10, especially Embodiment 7, it can be seen that the synergistic effect of PTFSI and LiBOB can construct a composite SEI. PTFSI can quickly react to generate LiF, lithium sulfonate and other products to provide high mechanical strength and ionic conductivity. LiBOB stabilizes the negative electrode SEI layer and can form an SEI film with moderate thickness. The synergy of the two further improves the battery performance, which is significantly better than the single additive system electrolyte. In addition, excessive PTFSI decomposition products may cause the internal resistance of the battery to rise, affecting the first cycle charge and discharge efficiency.

[0076] By comparing Examples 2, 5, 7, 9, and 10 with the remaining Examples, it can be seen that when the weight ratio of the first additive, the second additive, and the third additive is preferably (1-1.2):2:1, the formation speed and maturity of the SEI film can be more effectively balanced, so that the SEI film can be formed quickly to protect the electrode, but will not be too thick too early to increase the internal resistance of the battery, thereby achieving better results in the initial charge and discharge efficiency and cycle stability.

[0077] By comparing Example 7 with Examples 9 and 10, it can be seen that when the dosage ratios of the three additives are the same, the weight ratio of PTFSI to LiBOB in the first additive is preferably 1: (0.8-1), which can have a more significant synergistic effect in the SEI film formation process and ultimately further comprehensively improve the initial charge and discharge efficiency and cycle stability of the corresponding lithium-ion battery.

[0078] Comparing Example 1 with Examples 11-12, it can be seen that the reduction of DTD in the second additive will lead to insufficient sulfide in SEI, and the SEI crack resistance will decrease in long-term cycles, while increasing the DTD content will lead to more Li + Consumed to generate Li 2 SO 3 / Li 2 S, the irreversible capacity of the first cycle increases and the first efficiency decreases slightly.

[0079] Comparing Example 13 with Example 1, it can be seen that Example 13 changes the solvent ratio and reduces the EC content, the overall conductivity of the electrolyte decreases, and the ion transfer rate is affected, which in turn has a certain impact on the first effect and cycle of the battery sample.

[0080] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those described herein, for example.

[0081] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An electrolyte adapted for a dry electrode sheet, comprising an organic solvent and a lithium salt, characterized in that: Taking the total weight of the electrolyte adapted for the dry-process electrode sheet as 100%, the electrolyte adapted for the dry-process electrode sheet further comprises 1% to 5% of a first additive, 2% to 6% of a second additive, and 1% to 3% of a third additive; The lithium salt is LiPF6; The LUMO energy levels of the first additive, the second additive, the third additive and the LiPF6 are arranged as follows: the first additive < the second additive < the LiPF6 < the third additive.

2. The electrolyte adapted for dry-process electrode sheet according to claim 1, characterized in that: The first additive is selected from one or more of electrolyte additives and / or sulfonimide additives; The second additive is selected from one or more organic ester additives; The third additive is selected from one or more electrolyte additives.

3. The electrolyte adapted for dry-process electrode sheet according to claim 2, characterized in that: The weight ratio of the first additive, the second additive and the third additive is (0.5-2.5):2:

1.

4. The electrolyte adapted for dry-process electrode sheets according to any one of claims 1 to 3, characterized in that: The first additive is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium diethyl oxalate, N-phenylbis(trifluoromethanesulfonyl)imide, N-methylbis(trifluoromethanesulfonyl)imide and N-ethylbis(trifluoromethanesulfonyl)imide.

5. The electrolyte adapted for dry-process electrode sheets according to any one of claims 1 to 3, characterized in that: The second additive is selected from one or more of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, di(alkyne) oxalate and methylene methanedisulfonate.

6. The electrolyte adapted for dry-process electrode sheets according to any one of claims 1 to 3, characterized in that: The third additive is selected from one or more of lithium difluorophosphate, lithium chloride, lithium phosphate and lithium carbonate.

7. The electrolyte adapted for dry-process electrode sheets according to any one of claims 1 to 3, characterized in that: The organic solvent is selected from one or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate and butylene carbonate.

8. A method for preparing an electrolyte adapted for a dry-process electrode sheet according to any one of claims 1 to 7, characterized in that: include: Step S1, preparing a lithium salt solution by mixing the organic solvent and the lithium salt; Step S2, adding the first additive to the lithium salt solution, and obtaining a first mixed solution after a first stirring; Step S3, adding the second additive and the third additive to the first mixed solution, and obtaining the electrolyte adapted to the dry-process electrode sheet after a second stirring.

9. The method for preparing an electrolyte adapted for a dry-process electrode sheet according to claim 8, characterized in that: The first stirring speed is 200 rpm to 300 rpm, and the time is 20 min to 30 min; and / or, The second stirring has a rotation speed of 400 rpm to 500 rpm and a time of 30 min to 40 min.

10. A lithium ion battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, characterized in that: The electrolyte is an electrolyte adapted for dry-process electrode sheets as described in any one of claims 1 to 7, and both the positive electrode sheet and the negative electrode sheet are dry-process electrode sheets.

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